Method for the plasma remelting of a consumable metal bar in a controlled atmosphere

ABSTRACT

A method for the plasma-arc remelting of a consumable metal bar into ingots under controlled atmospheric conditions in which remelting is effected through the use of plastmatrons which serve for heating nitrogen gas to the plasma-state and feeding it together with activated metal for its remelting and alloying, and whereby an ingot is preformed in a cooled mould which is coupled to a power source.

United States Patent 1 Lakomsky et al.

51 May 29, 1973 [54] METHOD FOR THE PLASMA REMELTING OF A CONSUMABLE METAL BAR IN A CONTROLLED ATMOSPHERE [22] Filed: June 14,1971

211 App]. Nd; 152,556

{52]- U.S. Cl ..l3/34, 13/1, 75/10 R [5 l] lnt. Cl. ..H05b 7/00 [58] Field of Search ..13/31, 1,9, 34;

{56] References Cited UNITED STATES PATENTS 2/1970 Dukelow et a. ..219 121 P Primary Examiner-Roy N. Envall, Jr. AttarneyHolman & Stern [57] ABSTRACT A method for the plasma-arc remelting of a consumable metal bar into ingots under controlled atmospheric conditions in which remelting is effected through the use of plastmatrons which serve for heating nitrogen gas to the plasma-state and feeding it together with activated metal for its remelting and alloying, and whereby an ingot is preformed in a cooled mould which is coupled to a power source.

3 Claims, 1 Drawing Figure METHOD FOR THE PLASMA REMELTING OF A CONSUMABLE METAL BAR IN A CONTROLLED ATMOSPHERE The present invention relates to electrical metallurgy ,and has application in the production of ingots of highquality steel and alloys alloyed with nitrogen.

DESCRIPTION OF PRIOR ART Most commonly used at present for the production'of ingots is the method melting of steel and its alloys in' electric-arc and induction furnaces fitted with ceramic crucibles.

In the known methods nitrogen is added to the mol ten steel contained in a crucible in the form of nitrogenous additions, comprising, ferrocrome, manganese, etc. and the liquid steel thus produced in the crucibles, is then subject to teeming.

However, the foregoing method is associated with certain drawbacks.

In producing alloy steels containing nitrogen, the aforesaid nitrogenous additions should be prepared beforehand, and this tends to increase the cost of the steel obtained.

In addition, the above technique is applicable to the production of the ingots with only a restricted nitrogen content not exceeding the equilibrium nitrogen concentration in the liquid metal at the melting temperature and gas pressure equal to the atmospheric value, i.e. up to 0.6 percent depending on the content of alloying elements in the ingot, and this does not enable the production of high-tensile austenitic stainless steels.

To produce high-nitrogen steels both melting of metal in a furnace and pouring in moulds are carried out in high-pressure autoclaves and this requires more intricate technological production techniques and consequently increases the price of steel.

Also known is a method of the plasma-arc remelting of a consumable metal bar under controlled atmospheric conditions, in which remelting is accomplished 'by means of a plasmatron, whose function is to heat the gas to a plasma state and pass it to the metal to be heated, and an ingot is produced in a cooled mould, connected to a power supply (see FRG US. Pat. application No. 17584837).

With the above method the gas converted to a plasma state is employed as a source of heat.

However, the foregoing procedure is suitable only for the electroslag refining of a consumable bar.

It is an object of the present invention to develop a method of plasma-arc remelting which would make it possible both to increase the nitrogen content in the metal being remelted and to produce dense high- I nitrogen ingots.

The specified as well as other objects are achieved in the proposed method of plasma-arc remelting of a consumable metal bar under controlled atmosphere conditions.

Proposed herewith is a method of the plasma-arc remelting of a consumable metal bar in a controlled atmosphere in which the remelting is carried out with the aid of plasmatrons which serve for heating the gas to a plasma state and projecting a plasma torch onto the metal to achieve its heating, and an ingot is formed in a cooled mould which is coupled to a power source. According to the present invention, passed to the metal together with the heating plasma is nitrogen activated in the plasma and alloying the metal being remelted.

For producing high-nitrogen ingots without impairing their density it would be expedient to accomplish the remelting process in a plasma-arc furnace at a gas pressure ranging from 1 to 30 atm.

It would be also sound practice to introduce nitrogen in the zone of a plasma torch outside the plasmatron and this would increase durability of plasmatrons and provide for obtaining more steady plasma arcs.

To preclude the possibility of the nitrogen transfer from the metal to the atmosphere of a plasma-arc furnace it is reasonable to feed a layer of flux on the surface of molten metal enclosed in the mould.

A technique for the plasma-arc remelting of a com sumable metal bar under controlled atmospheric conditions derived from the present invention would enable the increase in nitrogen content in the metal to be remelted and the production of dense high-nitrogen ingots.

To make the present invention more clear below is given an exemplary embodiment to be considered in conjunction with the accompanying drawing, which is a diagrammatic longitudinal sectional view of a furnace for carrying into production the method of the plasmaarc remelting of a consumable metal bar in a controlled atmosphere, conforming to the present invention.

Consumable metal bar 1 (see FIG.) is mounted in the furnace chamber 2. The air is evacuated with the aid of pump 3. Then chamber 2 is filled with a mixture of insert gas and nitrogen taken in specified proportions whereupon the requisite pressure is set in the chamber 2 and gas to be heated is passed to plasmatrons 4 connected to power source 5. Nitrogen added to the metal to be remelted as an alloying element, is supplied through device 6, arranged in the wall of chamber 2, into the zone of a plasma torch 7 located outside of plasmatron 4.

After liquid metal bath 8 has been produced by remelting bar 1 both gears are turned on: bear 9 intended for feeding bar 1 and gear 10 for drawing out ingot 11.

The gas atmosphere contained in chamber 2 of the furnace can be controlled by varying either thecomposition or the pressure of the gas used.

To prevent the transfer of nitrogen from the metal into the atmosphere in a plasma-arc furnace, liquid bath 8 enclosed in mould 12 is coated with flux fed by weigh hopper 13.

The method of the invention has been used for the remelting of steel bars into ingots formed in a copper mould fitted with a cooling system.

The remelting process has been carried out in a nitrogenous atmosphere in the zone of the plasma torch located outside the plasmatron.

The gas pressure in the furnace, upon which the per-' centage of nitrogen in the ingots produced is dependent, is varied from 1 to 30 atmospheres during remeltmg.

These trial heats have proved that nitrogen activated in a stream of plasma tends to alloy the metal at a high rate.

Typical of the ingots produced by remelting by the method of the invention have been a dense structure and a preset nitrogen content. Maximum nitrogen content in the above ingots has been from 5 to 6 times as great as nitrogen concentration values common with the steel produced in the pots of induction and are furnaces by the conventional melting procedures with the consequent teeming of metal in the air.

The surface of the ingots produced according to the method of the invention was smooth and free from fissures; and, shrink holes were absent.

We claim:

1. In a method for plasma-arc remelting of a consumable metallic bar into an ingot having an increased nitrogen content including positioning the consumable metallic bar over a cooled mold, providing at least one plasmatron over the cooled mold, connecting an electrical power source between the mold and the plasmatron and remelting the consumable metallic bar whereby drops of molten metal forms a pool of molten metal in the cooled mold, the improvement comprising the steps of: providing a furnace chamber above the melt, which chamber is in sealing engagement with the plasmatron and consumable metallic bar to contain a controlled atmosphere; and introducing nitrogen into said chamber at a point removed from the plasmatron, which nitrogen alloys with the drops of molten metal to form the ingot with the increased nitrogen content.

2. The method as claimed in claim 1 including the further step of increasing the gas pressure in the furnace chamber up to atmospheres.

3. The method as claimed inclaim 1 including the further steps of introducing a flux onto the surface of the pool of molten metal to prevent nitrogen alloyed with the molten metal from passing back into the controlled atmosphere in the furnace chamber. 

2. The method as claimed in claim 1 including the further step of increasing the gas pressure in the furnace chamber up to 30 atmospheres.
 3. The method as claimed in claim 1 including the further steps of introducing a flux onto the surface of the pool of molten metal to prevent nitrogen alloyed with the molten metal from passing back into the controlled atmosphere in the furnace chamber. 